filesystem is not experimental anymore since c++17

- filesystem is merged into the mainline ISO C++ standard
  since C++ 17.
- As the minimum requirement for the project to compile is
  mentioned as C++17, we can now safely remove the experimental
  filesystem and use them directly instead.
- This would eliminate the stdc++fs dependency when migrated to
  meson.

Tested By;
- Compiled in all sdks, and unit tests are passed.

Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>
Change-Id: Icd63297a533c56e2d1952e99e17ec71747bd0e39
diff --git a/util.cpp b/util.cpp
index c880fd6..63b8f7e 100644
--- a/util.cpp
+++ b/util.cpp
@@ -11,7 +11,7 @@
 #include <algorithm>
 #include <cstdlib>
 #include <cstring>
-#include <experimental/filesystem>
+#include <filesystem>
 #include <fstream>
 #include <list>
 #include <nlohmann/json.hpp>
@@ -33,7 +33,7 @@
 
 using namespace phosphor::logging;
 using namespace sdbusplus::xyz::openbmc_project::Common::Error;
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 uint8_t toV6Cidr(const std::string& subnetMask)
 {